It is proposed to investigate the biosynthesis of heparin. A major objective is to determine loci of regulation of polymer assembly and to study the structural and functional organization of the enzymatic apparatus required for this process. Besides equipment for synthesis of a protein core, six glycosyltransferases are needed for polysaccharide chain formation, and five other enzymes catalyze a series of modifications which take place after initial assembly of the carbohydrate portion of the molecule. A thorough knowledge of these enzymes is required to understand the biosynthesis of heparin.